<template>
  <div>
    <p>num {{ num }}</p>
    <p>double1 {{ double1 }}</p>
    <input v-model="double2"/>
  </div>
</template>

<script>
export default {
  data() {
    return {
      num: 20,
    };
  },
  computed: {
    double1() {
      return this.num * 2;
    },
    double2: {
      get() {
        return this.num * 2;
      },
      set(val) {
        this.num = val / 2;
      },
    },
    // double3
  },
};
</script>
